1. Technical Field
The embodiments herein generally relate to wireless technologies, and, more particularly, to mobile television (TV) technologies.
2. Description of the Related Art
In mobile TV Digital Video Broadcasting over Handheld (DVB-H) systems such as DVB-H (ETSI EN 301 192), one radio frequency (RF) channel is shared among many TV channels (TV programs). These TV channels are multiplexed either in the time domain or in the frequency domain. When the TV channels are multiplexed in the time domain, each channel is given full access to the entire RF channel bandwidth for a short period of time (burst duration). After that burst is transmitted, bursts for other channels occupy the RF channel and so on. This multiplexing process is called time division multiplexing (TDM). FIG. 1 illustrates an example of time division multiplexing of 15 TV channels on one RF channel. The TV channels are labeled 1, 2, 3, . . . , 15. In FIG. 1, it is shown that each TV channel occupies the entire RF channel for 1/15 of the time. A receiver which is watching only one channel (for example, channel 2) needs only to be active (ON) during the periods of channel 2 bursts. In order to conserve battery consumption, such a receiver will typically shut off its circuits when channel 2 bursts are not occupying the RF channel. The receiver thus enters into a SLEEP mode. This shows that the TDM of channels can help reduce power consumption of a receiver watching a single channel.
On the other hand, this causes a problem when the user wants to switch to watch another TV channel on the same RF channel. One example is shown in FIG. 1, if the user wishes to switch to Channel 3 (denoted by Channel UP in FIG. 1). The worst case occurs when the user issues a command to switch to Channel 3 right after the burst of Channel 3 ends. In this case, the receiver has to wait until the next burst that belongs to Channel 3 appears on the RF channel. This causes the user to wait for a given period of time denoted as the channel switching delay. Such a delay could be as long as 5 to 7 seconds in DVB-H systems. Such channel switching delay could be rather annoying to the user. This delay is known as the delay in the physical layer or the PHY delay.
In addition to the PHY delay, there is an additional delay in the physical layer that arises from the nature of the video encoding process. Digital video encoding techniques normally treat a video signal as a stream of still pictures (video frames). The encoding process starts by encoding one frame independently of other frames. Such a frame is called the reference frame (I-frame). For the following frames, the video encoder finds the difference between each frame and the reference frame. The video encoder encodes only this difference in order to reduce the bit rate of the encoded video stream. In practice the size of a reference frame is significantly larger than a regular frame (P-frame). Video encoders transmit I-frames at a low rate that could be as low as one reference frame per second.
At the receiver, video frames are generally received in order. When the user switches to a given channel, the receiver waits for the first available reference frame to start decoding the video. The video decoder neglects all P-frames until it finds the first I-frame. After receiving the first I-frame, the video decoder can now decode all frames and the user can then start watching video. The amount of time between receiving a given channel and finding the first reference frame is a delay in the overall channel switching operation. The worst case delay depends on how often reference frames are transmitted. For example, if reference frames are transmitted once per second, then the worst case time delay is one second. In order to reduce such a delay, the transmitter has to transmit reference frames at a high rate, which could reduce the overall channel capacity as reference frames are significantly larger than normal frames. Accordingly, there remains a need for a new video encoding technique for mobile TV applications.